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Disk Scheduling and Management

Disk Scheduling and Management. Disk Request. A request must specify: Whether the operation is input or output, The disk address for the transfer, The memory address for the transfer, The size of the data block to be transferred (number of bytes).

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Disk Scheduling and Management

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  1. Disk Scheduling and Management CSCI 315 Operating Systems Design

  2. Disk Request A request must specify: • Whether the operation is input or output, • The disk address for the transfer, • The memory address for the transfer, • The size of the data block to be transferred (number of bytes). Disk requests are generated by processes. When the disk is busy serving a request, other incoming requests must be stored for later processing. For this purpose, the OS organizes these requests in a queue of pending requests for that disk unit. When the disk finishes serving a request, the OS much check the queue and if it is not empty, serve another request. What policy should be used for dealing with the queue? Disk Scheduling CSCI 315 Operating Systems Design

  3. FCFS Scheduling Consider the following sequence of requests where each number corresponds to a disk cylinder: 98, 183, 37, 122, 14, 124, 65, 67 queue = 98, 183, 37, 122, 14, 124, 65, 67 0 14 37 53 65 67 98 122 124 183 199 Question: How much does the disk head travel to serve these requests? Question: What metric could one define to characterize performance here? Question: How does this scheduling of requests affect the disk performance? CSCI 315 Operating Systems Design

  4. SSTF Scheduling 98, 183, 37, 122, 14, 124, 65, 67 queue = 65, 67, 37, 14, 98, 122, 124, 183 0 14 37 53 65 67 98 122 124 183 199 FCFS Total head movement = 640 SSTF Total head movement = 236 Question: If this is similar to SJF, does it share the same drawback? Question: Is the performance of SSTF any better than that of FCFS? Question: Is the performance of SSTF optimal? CSCI 315 Operating Systems Design

  5. SCAN Scheduling 0 14 37 53 65 67 98 122 124 183 199 98, 183, 37, 122, 14, 124, 65, 67 FCFS Total head movement = 640 SSTF Total head movement = 236 SCAN Total head movement = ? Assume the distribution of requests for cylinders is uniform. Consider the density of requests when the head reaches one end and reverses direction. Hmmm… few requests will be right in front of the head… The heaviest density will be at the other end of the disk and those requests will have waited the longest. Why not just go there first? CSCI 315 Operating Systems Design

  6. C-SCAN Scheduling 0 14 37 53 65 67 98 122 124 183 199 98, 183, 37, 122, 14, 124, 65, 67 When the head reaches the end of the disk, it immediately returns to cylinder 0. The algorithm essentially treats the cylinders as a circular list that wraps around from the final cylinder to the first one. CSCI 315 Operating Systems Design

  7. LOOK Scheduling 0 14 37 53 65 67 98 122 124 183 199 98, 183, 37, 122, 14, 124, 65, 67 Well, by now you have realized that scanning all the way to the extreme ends of the disk is perhaps wasteful. In practice, one could move the head only as far as the request that is farthest out. CSCI 315 Operating Systems Design

  8. C-LOOK Scheduling 0 14 37 53 65 67 98 122 124 183 199 98, 183, 37, 122, 14, 124, 65, 67 CSCI 315 Operating Systems Design

  9. Choosing a Disk Scheduling Algorithm • The criteria should involve fairness and performance. • Performance depends on the number and type of requests. • Given a string of cylinder references, one could find the optimal schedule to serve them all using, for instance, dynamic programming. Remember, however, that computing the optimal schedule takes time! • Another point to remember is that disk performance depends heavily on the file allocation method, on the location of directories and indices. • If disk scheduling is a separate OS module, it can easily be replaced. • Modern disk units don’t disclose the physical location of disk blocks, so it can be challenging to do scheduling in the OS. Disk controllers, however, can handle disk scheduling themselves, lessening the burden on the OS. CSCI 315 Operating Systems Design

  10. Disk Formatting A brand new magnetic disk is a blank slate. There is no such thing as sectors. Tracks exist only as abstractions: we know they are actually created by how disk heads move over the disk surface (step motors). Before a disk unit can be used as we’ve discussed, it is necessary to divide each track into sectors, what is known as low-level formatting. This formatting operation fills the disk with a special data structure for each sector: 11011001011 0111010100100101001 0111010100100101110101001 11010111010100100101110101001 00101100111010100100101110101001 1101010101011010100100101110101001 100101010101011010100100101110101001 0001101010101011010100100101110101001 00001101010101011010100100101110101001 0110110010101011010100100101110101001 1011101010101011010100100101110101001 1101010101011010100100101110101001 01010101011010100100101110101001 10000111010100100101110101001 0111010100100101110101001 01110101001001011101 111011001011 Disk surface header data trailer error-correcting code CSCI 315 Operating Systems Design

  11. Disk Formatting sector header data trailer sizeof(header)+sizeof(trailer) = overhead 256 512 1,024 sizeof(data)= track Larger sectors => less disk space used for overhead. Question: What is the potential drawback? Organization of a disk surface CSCI 315 Operating Systems Design

  12. Disk Formatting A disk can be further subdivided into partitions: a contiguous group of cylinders. Each partition is viewed by the operating system as a logical disk. Next, we’ll look at the partition table from a running Linux installation. CSCI 315 Operating Systems Design

  13. Example: Partition Table root# sbin/fdisk /dev/hdaThe number of cylinders for this disk is set to 4864.There is nothing wrong with that, but this is larger than 1024,and could in certain setups cause problems with:1) software that runs at boot time (e.g., old versions of LILO)2) booting and partitioning software from other OSs(e.g., DOS FDISK, OS/2 FDISK)Command (m for help): pDisk /dev/hda: 40.0 GB, 40007761920 bytes255 heads, 63 sectors/track, 4864 cylindersUnits = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System/dev/hda1 1 4 32098+ de Dell Utility/dev/hda2 * 5 3064 24579450 c Win95 FAT32 (LBA)/dev/hda3 3065 3701 5116702+ 83 Linux/dev/hda4 3702 4864 9341797+ f Win95 Ext'd (LBA)/dev/hda5 3702 3832 1052226 82 Linux swap/dev/hda6 3833 4864 8289508+ 83 Linux CSCI 315 Operating Systems Design

  14. Logical Formatting Before a partition can be used, a file-system needs to be created on the partition. That means, file-system data structures are written to the disk. Many different types of file-systems exist: ext2, ext3, swap, FAT, FAT32, ISO9660, etc. Read the man page of the mount command on Linux or Solaris for more information. One may be able to use a partition as just a sequential array of logical blocks bypassing file-system data structures: this is called raw I/O. It also bypasses buffer caches, file locks, pre-fetching, file names, space allocation, and directories. CSCI 315 Operating Systems Design

  15. File System Types in Linux Excerpt from the mount man page: The argument following the -t is used to indicate the file sys- tem type. The file system types which are currently supported are: adfs, affs, autofs, coda, coherent, cramfs, devpts, efs, ext, ext2, ext3, hfs, hpfs, iso9660, jfs, minix, msdos, ncpfs, nfs, ntfs, proc, qnx4, ramfs, reiserfs, romfs, smbfs, sysv, tmpfs, udf, ufs, umsdos, vfat, xenix, xfs, xiafs. Note that coherent, sysv and xenix are equivalent and that xenix and coherent will be removed at some point in the future — use sysv instead. Since kernel version 2.1.21 the types ext and xiafs do not exist anymore. For most types all the mount program has to do is issue a simple mount(2) system call, and no detailed knowledge of the filesys- tem type is required. For a few types however (like nfs, smbfs, ncpfs) ad hoc code is necessary. The nfs ad hoc code is built in, but smbfs and ncpfs have a separate mount program. In order to make it possible to treat all types in a uniform way, mount will execute the program /sbin/mount.TYPE (if that exists) when called with type TYPE. Since various versions of the smbmount program have different calling conventions, /sbin/mount.smb may have to be a shell script that sets up the desired call. CSCI 315 Operating Systems Design

  16. Boot Block The system’s primary disk unit contains a boot block that contains the bootstrapping program that loads the OS to memory. This program is invoked by the computer’s minimal bootstrap program in ROM. This boot block is often called the Master Boot Record (MBR). Different operating systems treat the MBR in very different ways. Some a flexible enough to install a boot loader in the MBR, so that the disk can contain different OS in different disk partitions. The loader for each OS is then stored at the beginning of its own partition. Examples: Windows NT/2000/xp boot loader, Linux lilo and grub. A “bootable” disk is one on which a boot block has been installed. CSCI 315 Operating Systems Design

  17. Bad Block Management Certain disk blocks can’t be used reliably due to problems with the magnetic media. Rather than just junk the entire disk, one can just mark the bad blocks and ignore them in the allocation procedures. If blocks go bad after formatting, a disk check application can take care of marking them off (i.e. chkdsk in MS-DOS). The data in marked blocks is irretrievably lost. CSCI 315 Operating Systems Design

  18. Swap Space Management Swap space can be allocated in two different ways: 1) From a single file in the normal file system. Normal file operations are used to manipulate this file. External fragmentation can become a problem requiring many seeks. 2) From a swap partition. This can be much faster than (1), but internal fragmentation can be a problem. This problem is relatively small considering that the lifetime of files on the swap space is small. Adding more swap, however, is not easy since it requires repartitioning the disk. CSCI 315 Operating Systems Design

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